Page 379 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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LATERAL-FORCE DESIGN

LATERAL-FORCE DESIGN 8.33

Eccentrically braced frames are a rational attempt to design steel structures that fully develop the
ductility of the steel without loss of strength and stiffness due to buckling. The design of these frames
is somewhat more complicated than that of some other steel frames, but eccentric braced frames
offer advantages in economical use of steel and seismic performance that cannot be duplicated by
other systems.

8.7.4 Limitations on Buckling-Restrained Braced Frames
Buckling-restrained braced frames, illustrated in Fig. 8.11, are a relatively new seismic design con-
cept. These items are patented products provided by a number of different suppliers. The axial force-
deformation behavior of these braces is very good, as illustrated in Fig. 8.7d. However, the
performance of the system depends on many factors in addition to the buckling-restrained brace
itself. Buckling-restrained braces are being used with increasing frequency, and are treated in the
2005 AISC seismic provisions. It is likely that more substantial recommendations will be included
in future editions of these provisions, as the current provisions require extensive testing to document
the performance of the buckling-restrained brace, and the structural system.
Buckling-restrained braces are typically a flat bar or cruciform section encased within a steel
shell, as depicted in Fig. 8.11. The flat bar is not bonded to the encasing element, but the encasing
element is stiff enough to prevent buckling of the bar in compression. Because the axial bar yields
in both axial tension and compression, significant strain hardening is expected. These strain-
hardening values vary from system to system, but the strain hardening is commonly between
30% and 50% of the yield resistance. Because the strain hardening places additional demands on
columns and connections, the AISC seismic provisions require that the strain hardening be accu-
rately estimated for the buckling-restrained bracing system to obtain an adjusted brace strength.
The columns and gusset plate connections must then be designed for the expected maximum brace
strength including the strain hardening. Testing and verification of the performance of the buckling-
restrained brace is currently required, but in many cases the manufacturers may have performed
tests that provide adequate verification.
While most testing of buckling-restrained braces has been performed on individual brace ele-
ments, very recent tests have shown that system behavior may be different from that implied by tests
performed on the brace only. This occurs primarily because of the stiff gusset plate connections that
result with this bracing system, and the frame restraint that is produced by this connection stiffness.
Research is in progress to improve connection design criteria, and engineers should be continually
aware of new developments in this area.

8.8 FORCES IN FRAMES SUBJECTED TO LATERAL LOADS

The design loads for wind and seismic effects are applied to structures in accordance with the guide-
lines in Arts. 8.2 to 8.5. Next, the structure must be analyzed to determine forces and moments for
design of the members and connections. Member and connection design proceeds quite normally for
wind-load design after these internal forces are determined, but seismic design is also subject to the
detailed ductility considerations described in Arts. 8.6 and 8.7. Today, steel frames are nearly always
designed with the aid of a computer analysis to consider the frame stiffness, deformation, and dis-
tribution of forces. However, approximate analysis methods are desirable for preliminary analysis
and initial member sizing needed prior to development of computer models. Two such methods for
frames subject to lateral loads are the portal and cantilever methods.
The portal method is used for buildings of intermediate or shorter height. In this method, a bent is
treated as if it were composed of a series of two-column rigid frames, or portals. Each portal shares one
column with an adjoining portal. Thus, an interior column serves as both the windward column of one
portal and the leeward column of the adjoining portal. Horizontal shear in each story is distributed in
equal amounts to interior columns, while each exterior column is assigned half the shear for an inte-
rior column, since exterior columns do not share the loads of adjacent portals. If the bays are

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